Various over-the-air interfaces exist for wireless communication systems. In connection with these interfaces, various standards have been established including, for example, Global System for Mobile Communications (GSM). Transmitters used in these wireless communication systems to transmit voice over these interfaces can cause interference when used in close proximity to sound reproducing equipment such as microphones. The transmitter pulse repetition rate can be detected in the microphone giving rise to an audible buzz to users. Such undesired transmitter detection can occur by the microphone in a cellular phone or by the microphone in a headset being used in conjunction with the cellular phone. In the prior art, cellular phones and headsets utilize discrete capacitors and inductors to filter the unwanted radio frequency (RF) interference resulting from microphone detection of the transmitter pulse repetition rate. Such capacitive RF interference filters are well known in the art.
Microphone assemblies used in telephonic devices and headsets include a microphone transducer, sound port, and a housing containing the signal processing circuitry. The microphone transducer is typically an electret type microphone comprised of a charged diaphragm forming one plate of the capacitor and a backplate forming the other terminal. Sound impinging on the diaphragm varies the capacitance and produces a voltage signal proportional to the sound waves which is picked off the backplate and coupled to signal processing circuits. Such electret microphones typically use a Junction Field Effect Transistor (JFET) sensitive to RF interference. The JFET is necessary to transform the very high impedance of the small capacitor formed by the electret microphone to a more usable value of a few thousand ohms.
Electret microphones, which contain a sensitive active component FET, suffer from radio frequency (RF) interference when used in proximity to RF transmitters. As a result of the proximity of microphones and RF transmitters in cellular or cordless phones and headsets, RF interference problems frequently arise and must be addressed. RF interference problems manifest themselves as audio noise on the microphone output, resulting in objectionable background audio noise during user communication. In Global System for Mobile Communications (GSM) systems, the problem is often referred to as GSM “buzz” to those of skill in the art. GSM buzz results when the transmitter's RF pulse repetition rate is sufficiently low to be detected by the microphone and demodulated by an active component field effect transistor in the microphone circuitry, producing an audible buzz in the output of the microphone resulting from RF interference caused by operation of the transmitter in a GSM system. In IDEN modulation systems, RF interference manifests as “chopper noise”, a term used in the art to refer to the helicopter like sound of the resulting audio interference due to unwanted demodulation of IDEN signals.
In the prior art, radio frequency interference problems in the headset and cell phone industry is addressed in part by the use of discrete RF filter capacitors components which are placed on the microphone PCB near the FET to filter radiated and conducted RF energies from the sensitive FET. Additional discrete components may be utilized as well, including shielding, inductors, and capacitors.
Problems with the prior art solutions include additional expense of the discrete components used, potential unreliability and irregularity of off-the-shelf discrete components. Furthermore, use of discrete filter capacitors and components require valuable space on the printed circuit board (PCB) and labor to install. Thus, there has been a need for improvements in the filtering of unwanted RF interference in microphone designs. More specifically, there has been a need for improved microphone printed circuit boards that provide effective filtering and attenuation of RF interference.